1. Field of the Invention
The present invention relates to machines for treatment of the surface of round timber, particularly such machines including at least one arm adapted to perform a rotational movement around the timber and to be swingable towards the timber during such movement in response to spring forces exerted on the arm. Specifically the invention relates to the combination of such an arm and a working tool removably attached to the free end of the arm.
2. Description of the Prior Art
U.S. Pat. No. 3,189,067 discloses a barking machine of the hollow rotor type having barking tools in the form of swingable arms. The barking part of the tool--the barking edge--is provided on a removable and easily replaceable element.
Through said U.S. Patent it is previously known also to locate not only the barking edge on such an element, but also the terminating and most wear exposed part of the edge of the swingable arm. This latter edge, the climbing edge, automatically makes the arm swing out to operative, barking position at contact with the leading end of timber fed through the rotor.
The mode of operation of the barking means of machines of the above kind is substantially as follows. The barking edge at the free end of the swingable arm is parallel to the longitudinal axis of the timber to be barked. This edge may be sharp but is generally more or less blunt. The barking edge is pressed against the surface of the timber under a spring force imparted by the swingable arm, which force is so chosen that the edge part of the arm penetrates the bark but not the wood surface. As the timber, or log, is fed in its longitudinal direction through the rotating rotor, this latter brings the swingable arm along, so that its edge moves in a helical path along the surface of the timber. Under the influence of the spring force directed towards the timber surface the edge is held in a continuous and sliding contact with the wood surface and loosens the bark encountered on the helical path from the wood, pushes the bark ahead in the rotational direction and throws it out of the rotor. This barking operation is made possible by the fact that bark and wood is kept together by a thin zone of minor strength, the so called cambium layer.
Loosening of the bark generally occurs by the force of the barking means in tangential direction resulting in shear stresses in the cambium layer exceeding the shear strength thereof. This kind of barking often is referred to as tangential shear barking and can also be described such that the barking tool is a scraper, the edge of which is kept in elastic contact with the comparatively hard wood surface and in tangential direction scrapes the soft bark off the round timber which is fed in axial direction through the barking machine.
Of importance for the barking result is the angle between tangential planes to the timber through the barking edge and the scraping or shearing plane of the tool; this angle is referred to as the "attacking angle".
The detachable and easily replaceable element shown in the drawings of U.S. Pat. No. 3,189,067 is constituted by a comparatively thick, rectangular plate. At the centre of one face the plate is provided with a cylindrical bolt. The plate rests againt a plane which is milled down in the swingable arm close to the free end thereof. In the said plane is a cylindrical recess wherein the cylindrical bolt fits. The plate is attached to the swingable arm by a screw passing through a bore in the arm. At tightening of the screw, however, the friction between the plate and the supporting plane-milled surface does not become so high that it cannot be ascertained that the plate does not rotate about the cylindrical bolt when the plate is exposed to forces in connection with barking and climbing. Hence, the plate is fixed by its edge facing the swinging axis of the arm being adapted to abut the shoulder extending across the arm, this shoulder resulting from the milling of the seat for the detachable element.
Detachable tips for barking tools corresponding to the description above have come to common use in barking machines of the hollow rotor type during the last ten year period. Although the maintenance of the active parts of the barking means has been facilitated hereby, the barking properties of such tools have been deteriorated in comparison with tools having a stationary tip. This is because the detachable tips hitherto used in this art have caused an increase of mass in the end portion of the arm in comparison with the case when stationary tools are employed. Thereby the inertia of mass of the swingable arm system has been remarkably increased. At unchanged revolution speed of the rotor and unchanged thrust force of the barking edge against the timber, the acceleration of the barking edge under the influence of the elastic press force becomes slower. Thereby the possibilities of the edge to follow the unevennesses of the wood surface diminish and this leads to a reduction of barking quality.
By lowering the speed of the rotor the barking quality can be improved and the increase of inertia of mass of the swingable arm system can be compensated for. This improvement, however, occurs at the cost of decreased capacity, since the possible feeding speed in the axial direction is proportional to the rotor speed. In order to achieve the self-evident characteristic of high capacity and good quality, a low inertia of mass is a significant and desirable factor.
Available detachable elements are usually produced by precision casting. The barking and climbing edges of such an element are coated with wear resistant alloys by welding or with hard metal plates by soldering. The mechanical strains on the plate, as well as thermal strains due to repeated hard metal coatings, cause a need to make the plate rigid, i.e. thick and heavy.
In order to provide a standardized swingable arm, provided with a likewise standardized plate seating, with a more forcefully scraping (aggressive) tool, such as the one described in U.S. Pat. No. 2,880,771, detachable tips are produced which are not plane-parallel but have increased thickness towards the barking edge. Such detachable tips having wedge shaped longitudinal sections have a greater mass than the normal plane-parallel detachable tips.
The wish to fasten the detachable tip with just one screw makes the precision achievable by casting insufficient. For replaceability and function, thus, not only the plate-seat with its cylindrical bore in the swingable arm but also the under-face and cylindrical bolt of the plate and its edge facing the shoulder of the seat must be worked with close tolerances. It is not possible to make such detachable elements turnable with two barking and two climbing edges; after turning the worn barking edge would not fit accurately enough against the shoulder of the plane-milled surface to prevent rotation and loosening of the element when it is used for its purpose. Detachable elements of this kind become so expensive in manufacture that non-recurring use hardly can be contemplated. Thus, the elements are detached from the swingable arms and are maintained in the same way as tools with stationary tips, i.e. by repeated hard metal coating and re-grinding of barking edge as well as climbing edge.
The object of the present invention is to provide turntable and light detachable tips for swingable arms in barking machines of the hollow rotor type and to shape the free end portion of the arms such that said portion together with a detachable tip form a single and easily detachable, but still secure joint. Detachable tips according to the invention are so cheap in manufacture that they can be discarded after use instead of being subjected to conventional normal hard metal coating and re-grinding operations. Detachable tips according to the invention are primarily intended for barking but may in combination therewith also be utilized for the automatic opening of the swingable arms. The invention is also applicable to cutting knives where the primary purpose is not to bark but to cut a helical notch through the bark down to the wood surface with assist of the hollow rotor machine. By means of the present invention the drawbacks of the known detachable tips set forth above are eliminated.